The present invention relates to a laser beam irradiation method and a laser irradiation apparatus for using the method (apparatus including a laser and an optical system for guiding laser beam emitted from the laser to an object to be irradiated). In addition, the present invention relates to a method of manufacturing a semiconductor device, which includes a laser beam irradiation step. Note that a semiconductor device described here includes an electro-optical device such as a liquid crystal display device or a light-emitting device and an electronic device that includes the electro-optical device as a part.
In recent years, a wide study has been made on a technique in which laser annealing is performed for a semiconductor film formed on an insulating substrate made of glass or the like, to crystallize the film, to improve its crystallinity so that a crystalline semiconductor film is obtained, or to activate an impurity element. Note that a crystalline semiconductor film in this specification indicates a semiconductor film in which a crystallized region is present, and also includes a semiconductor film that is crystallized as a whole.
A method of forming pulse laser beam from an excimer laser or the like by an optical system such that it becomes a square shape or a linear shape on an irradiation surface, and scanning the laser beam (or relatively shifting an irradiation position of the laser beam with respect to the irradiation surface) to conduct annealing is superior in mass productivity and is excellent in technology. The xe2x80x9clinear shapexe2x80x9d described here means not a xe2x80x9clinexe2x80x9d in the strict sense but a rectangle (or a prolate ellipsoid shape) having a high aspect ratio. For example, it indicates a shape having an aspect ratio of 10 or more (preferably, 100 to 10000). Note that the linear shape is used to obtain an energy density required for sufficiently annealing an object to be irradiated. Thus, if sufficient annealing is conducted for the object to be irradiated, it may be either a rectangular shape or a planar. Presently, excimer lasers with 15 J/pulse come onto the market and there is a possibility to perform a laser anneal by a planar beam. Further, the spot of the laser light is made laser light""s energy distribution on an irradiation surface of the laser light when there is not a special definition.
FIGS. 10A and 10B show an example of a configuration of an optical system for forming laser beam in a linear shape on an irradiation surface. This configuration is extremely general. All optical systems described above are based on the configuration shown in FIGS. 10A and 10B. According to the configuration, a cross sectional shape of laser beam is converted into a linear shape, and simultaneously an energy density distribution of laser beam on the irradiation surface is homogenized. In general, an optical system for homogenizing the energy density distribution of laser beam is called a beam homogenizer.
The spot of the laser beam emitted from a laser 71 is divided by a cylindrical lens array 73. The direction is called a first direction in this specification. It is assumed that, when a mirror is inserted in a course of an optical system, the above-mentioned first direction is changed in accordance with a direction of light bent by the above-mentioned mirror. In this configuration, the cylindrical lens array is divided into seven parts. Then, the laser beams are superposed on an irradiation surface 79 by a cylindrical lens 74, thereby homogenizing an energy density distribution of the linear laser beam in the longitudinal direction, and the length of the longitudinal direction is determined.
Next, the configuration shown in the side view of FIG. 10B will be described. The spot of the laser beam emitted from a laser 71 is divided by cylindrical lens arrays 72a and 72b. The direction is called a second direction in this specification. It is assumed that, when a mirror is inserted in a course of an optical system, the second direction is changed in accordance with a direction of light bent by the mirror. In this configuration, the cylindrical lens arrays 72a and 72b each are divided into four parts. The divided laser beams are temporarily synthesized by a cylindrical lens 74. After that, the laser beams are reflected by a mirror 77 and then condensed by a doublet cylindrical lens 78 so that they become again single laser beam on the irradiation surface 79. The doublet cylindrical lens 78 is a lens composed of two cylindrical lenses. Thus, an energy density distribution of the linear laser beam in a width direction is homogenized, thereby homogenizing an energy density distribution of the linear laser beam in the longitudinal direction, and the length of the width direction is determined.
For example, an excimer laser in which a size in a laser window is 10 mmxc3x9730 mm (which each are a half-width in beam profile) is used as the laser 71 and laser beam is produced by the optical system having the configuration shown in FIGS. 10A and 10B. Then, linear laser beam which has a uniform energy density distribution and a size of 125 mmxc3x970.4 mm can be obtained on the irradiation surface 79.
At this time, when, for example, quartz is used for all base materials of the optical system, high transmittance is obtained. Note that coating is preferably conducted for the optical system such that transmittance of 99% or more is obtained at a frequency of the used excimer laser.
Then, the linear laser beam formed by the above configuration is irradiated with an overlap state while being gradually shifted in a width direction thereof. Thus, when laser annealing is performed for the entire surface of an amorphous semiconductor film, the amorphous semiconductor film can be crystallized, crystallinity can be improved to obtain a crystalline semiconductor film, or an impurity element can be activated.
At an edge of linear, rectangular, or sheet-like laser light formed on an irradiation surface or in the vicinity thereof by an optical system, the energy density is attenuated gradually due to aberration of a lens or the like (FIG. 11A). In this specification, a region at a laser light edge where the energy density is gradually attenuated is called an attenuation region.
As the substrate area is increased and the laser power is raised, it is now possible to form a longer linear beam or rectangular beam and a larger sheet-like beam. Annealing with such laser light is more efficient. However, the energy density of laser light emitted from a laser is smaller at its edge than around the center. Therefore, if laser light is expanded by an optical system more than prior art, attenuation in the attenuation region is intensified.
In the attenuation region, the energy density is lower than a region having a high uniformity in energy density and the low energy density is attenuated gradually. For that reason, an irradiation object cannot be annealed uniformly by laser light that has the attenuation region (FIG. 11B). Even when the laser light scans an irradiation object for annealing in a manner that makes the attenuation regions overlap each other, it still is impossible to anneal the irradiation object uniformly because annealing conditions of the attenuation region are entirely different from annealing conditions of the highly uniform region. Accordingly, a region annealed by the attenuation region of laser light and a region annealed by the highly uniform region of the laser light cannot be treated equally.
For example, when a semiconductor film is an irradiation object, a region of the semiconductor film that is annealed by the attenuation region and a region of the semiconductor film that is annealed by the highly uniform region have different crystallinity. Therefore, if this semiconductor film varied in crystallinity from one region to another is used to manufacture TFTs, the electric characteristic of a TFT formed from the region that is annealed by the attenuation region is inferior to other TFTs and causes fluctuation among the TFTs on the same substrate.
As shown in FIG. 10, a complicate optical system is needed to form a linear beam. Optical adjustment for an optical system as such is very difficult to perform and, in addition, the apparatus has to be large in size because of large footprint.
If laser light used has high reflectance against an irradiation object and the laser light enters the irradiation object perpendicular to the object, the light goes back the light path it used upon entering the irradiation object (return light). Return light affects laser apparatus by changing the laser output and frequency and by breaking a rod.
The present invention has been made in view of the above, and an object of the present invention is therefore to provide laser irradiation apparatus which uses a simpler optical system than prior art to form a rectangular beam with the attenuation region at a laser light edge reduced for efficient annealing. Another object of the present invention is to provide a laser irradiation method using this laser irradiation apparatus as well as a method of manufacturing a semiconductor device which includes the laser irradiation method in its process.
The present invention is characterized in that laser beams emitted from a plurality of lasers are each divided and that laser beams emitted from different lasers and having different energy distributions are synthesized to form laser light having excellent uniformity in energy distribution. Here, energy distributions which are not identical but become the same distribution by rotation are deemed as different energy distributions. Also, the present invention is characterized in that laser beams emitted from a plurality of lasers are each divided, and that a laser beam including at least one laser beam that is emitted from a different laser and is in a different positional relation is synthesized with another such laser beam to form laser light having excellent uniformity in energy distribution.
Also, the present invention is characterized in that laser beams emitted from a plurality of lasers are each divided, and that laser beams emitted from different lasers and having different energy distributions enter a convex lens at an angle, exit the convex lens, and are synthesized on an irradiation surface or in the vicinity thereof to form rectangular laser light having excellent uniformity in energy distribution.
Even when laser beams emitted from different lasers are overlapped, they do not interfere each other. Accordingly the present invention is effective especially for laser light irradiation that uses highly interferential lasers such as a YVO4 laser having a coherent length of several tens to several hundreds m and a YAG laser having a coherent length of 1 cm or more.
By making laser light enter a convex lens at an angle, astigmatism or other aberration is caused to shape laser light into a linear shape on an irradiation surface or in the vicinity thereof.
When a divided laser beam is overlapped with another divided laser beam, it is preferred to overlap laser beams having different energy distributions from one another. This is because overlapping a large number of laser beams that have different energy distributions produces uniform laser light.
A structure of the present invention disclosed in this specification is laser irradiation apparatus characterized by comprising: a plurality of lasers; means for dividing each of plural first laser beams emitted from the plural lasers into plural second laser beams; and means for choosing one laser beam out of the second laser beams for each of the plural first laser beams and synthesizing the chosen second laser beams in the same region on an irradiation surface or in the vicinity thereof.
In the above structure, the laser irradiation apparatus is characterized in that the lasers are continuous wave or pulse oscillation solid-state lasers or gas lasers or metal lasers. Examples of the solid-state lasers include a continuous wave or pulse oscillation YAG laser, YVO4 laser, YLF laser, YAlO3 laser, Y2O3 laser, glass laser, ruby laser, alexandrite laser, and Ti:sapphire laser. Examples of the gas lasers include a continuous wave or pulse oscillation excimer laser, Ar laser, Kr laser, and CO2 laser. Examples of the metal lasers include a continuous wave or pulse oscillation helium cadmium laser, copper steam laser, and gold steam laser.
In the above structure, the laser light is desirably converted into harmonic by a non-linear optical element. For example, a YAG laser is known to output laser light having a wavelength of 1065 nm as the fundamental wave. This laser light is absorbed by a silicon film at a very low absorption coefficient and it is technically very difficult to crystallize an amorphous silicon film, one of semiconductor films, with this laser light. However, this laser light can be converted into a shorter wavelength by a non-linear optical element. Examples of harmonic thereof include the second harmonic (532 nm), the third harmonic (355 nm), the fourth harmonic (266 nm), and the fifth harmonic (213 nm). These harmonics are absorbed in an amorphous silicon film at a high absorption coefficient and therefore can be used in crystallization of an amorphous silicon film.
In the above structure, the laser irradiation apparatus is characterized in that the dividing means is one or more kinds selected from a slit, a mirror, a prism, a cylindrical lens, and a cylindrical lens array.
In the above structure, the laser irradiation apparatus is characterized in that the synthesizing means is one or more kinds selected from a mirror and a cylindrical lens.
A structure of the present invention disclosed in this specification is a method of laser irradiation, characterized by comprising: dividing each of plural first laser beams that are emitted from a plurality of lasers into plural second laser beams; choosing one laser beam out of the second laser beams for each of the plural first laser beams and synthesizing the laser beams in the same region on an irradiation surface or in the vicinity thereof.
In the above structure, the laser irradiation apparatus is characterized in that the lasers are continuous wave or pulse oscillation solid-state lasers or gas lasers or metal lasers. Examples of the solid-state lasers include a continuous wave or pulse oscillation YAG laser, YVO4 laser, YLF laser, YAlO3 laser, Y2O3 laser, glass laser, ruby laser, alexandrite laser, and Ti:sapphire laser. Examples of the gas lasers include a continuous wave or pulse oscillation excimer laser, Ar laser, Kr laser, and CO2 laser. Examples of the metal lasers include a continuous wave or pulse oscillation helium cadmium laser, copper steam laser, and gold steam laser.
In the above structure, the laser light is desirably converted into harmonic by a non-linear optical element.
A structure of the present invention disclosed in this specification is a method of manufacturing a semiconductor device, characterized by comprising: dividing each of plural first laser beams that are emitted from a plurality of lasers into plural second laser beams; choosing one laser beam out of the second laser beams for each of the plural first laser beams to obtain third laser beams and synthesizing the third laser beams in the same region on an irradiation surface or in the vicinity thereof to form a fourth laser beam; and irradiating a semiconductor film with the fourth laser beam while moving the laser beam relative to the semiconductor film.
In the above structure, the laser irradiation apparatus is characterized in that the lasers are continuous wave or pulse oscillation solid-state lasers or gas lasers or metal lasers. Examples of the solid-state lasers include a continuous wave or pulse oscillation YAG laser, YVO4 laser, YLF laser, YAlO3 laser, Y2O3 laser, glass laser, ruby laser, alexandrite laser, and Ti:sapphire laser. Examples of the gas lasers include a continuous wave or pulse oscillation excimer laser, Ar laser, Kr laser, and CO2 laser. Examples of the metal lasers include a continuous wave or pulse oscillation helium cadmium laser, copper steam laser, and gold steam laser.
In the above structure, the laser light is desirably converted into harmonic by a non-linear optical element.
In the above structure, the semiconductor film is desirably a film containing silicon. A glass substrate, a quartz substrate, a silicon substrate, a plastic substrate, a metal substrate, a stainless steel substrate, a flexible substrate, etc. can be used as a substrate on which the semiconductor film is formed. Examples of the glass substrate include a barium borosilicate glass substrate and an aluminoborosilicate glass substrate. A flexible substrate means a substrate in the form of a PET, PES, PEN, or acrylic film or other similar film. When a flexible substrate is used to manufacture a semiconductor device, the device can have a reduced weight. It is desirable to form on the front side, or on the front side and back side, of a flexible substrate a single layer or multi-layer of aluminum films (AlON, AlN, AlO, or the like), carbon films (DLC (diamond-like carbon) or the like), or SiN films as a barrier layer because the barrier layer improves the durability and other properties.
The present invention synthesizes laser beams emitted from different lasers on an irradiation surface or in the vicinity thereof and therefore interference does not take place. Most desirably, laser beams having different energy distributions from one another are synthesized on an irradiation surface or in the vicinity thereof. However, since the optimum synthesizing method varies from one laser light mode to another, the synthesizing method used can be chosen to suit individual cases. For example, laser light in the TEMoo mode is highly symmetrical and therefore laser light having a relatively high uniformity can be obtained by dividing laser light into two and synthesizing the left half and right half thereof. Needless to say, more highly uniform laser light is obtained when the number of division is larger. Laser light in other modes can also provide highly uniform laser light using the same method.
The present invention can irradiate a semiconductor film formed on a substrate with a rectangular beam having highly uniform energy distribution. Accordingly, a semiconductor film of uniform physical property can be obtained. This makes it possible to reduce fluctuation in electric characteristic of TFTs manufactured from this semiconductor film. This also improves the operation characteristic and reliability of a semiconductor device manufactured from these TFTs.